The present invention relates to a method and apparatus for protecting a mechanical press from overloading and, more particularly, to a non-destructive elastomeric overload apparatus.
Mechanical presses are widely used in the metalworking industry for numerous part forming operations and processes. The force or load developed by a mechanical press varies with the position of the slide assembly. Therefore, mechanical presses are generally protected against "overloaded" operations which occur when the force developed by the mechanical press is greater than its maximum rated capacity.
There are generally two types of overload devices, typically referred to as "permanent" and "replaceable", which are conventionally installed on mechanical presses. Both types of overload devices are adapted to protect against an "overload" occurring near the bottom of the stroke. Various causes exist for overloading of a mechanical press. For example, a small error on the part of a die-setter in adjusting the shut height of the slide assembly can result in an overload condition capable of fracturing one or more of the frame members or the crown of the press. Even when the damage is less severe, such as breakage of a crankshaft or an eccentric shaft, substantial bearing damage almost always occurs. Overloading can also be caused by double blanks, by misfeeds or from attempting to form parts in an under-capacity press. The latter cause is commonly experienced in sheet-metal forming operations (i.e., automotive body parts) since mechanical presses are frequently operated above their maximum tonnage rating in an effort to produce a superior contoured surface finish on the workpiece.
Conventional "permanent" overload devices include sensors built into the dies, and hydraulic actuators mounted on the slide adjustment which are adapted to trip a limit switch if a present hydraulic load is exceeded. Alternatively, friction clutches and slip-type couplings have been used to permit the drive mechanism to slip under severe overload.
Most "replaceable" overload devices take the form of either a stretch link or a shear collar. Stretch link devices are generally used on under-driven presses and consist of a link member that will yield (i.e., stretch) if a given load is exceeded. A shear collar is a metallic ring typically mounted on the slide adjustment mechanism which is designed to shear (i.e., fracture) when the mechanical press is overloaded. The shear collar has an area of reduced cross-section which is designed to fracture at a predetermined overloaded condition for providing additional displacement of the slide assembly near the bottom of the stroke to relieve the overloaded condition.
Disadvantages commonly associated with "replaceable" overload devices include: (a) they must be replaced after each overload occurrence; (b) they fail under less than rated load upon extended cyclical use; and (c) multiple devices installed on a multiple point slide assembly may not fail simultaneously and, therefore, may potentially cause severe side loading on the mechanical press. Therefore, use of "replaceable" overload devices typically results in lost productivity due to equipment down time.